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4f948b5949
bpf_binprm_select_interp() checks the name its load program passes with
strnlen(name, name__sz) and then hands the same buffer to
binfmt_misc_find_interp(), which compares it with an unbounded strcmp().
The buffer can be a BPF map value that another CPU rewrites between the
two reads. If the terminating NUL is overwritten in that window, strcmp()
reads past the name__sz bytes the verifier checked. That is an
out-of-bounds read of up to 31 bytes of whatever follows the checked
name__sz bytes.
The verifier checks the name and name__sz pair with BPF_READ | BPF_WRITE,
so a writable array map value is an accepted argument.
bpf(BPF_MAP_UPDATE_ELEM) on an array map copies the new value over the
old one in place and takes no lock. The NUL that strnlen() finds can be
overwritten before strcmp() reads the buffer again:
CPU0 CPU1
bpf_binprm_select_interp()
strnlen(name, name__sz)
finds the NUL inside name__sz
bpf(BPF_MAP_UPDATE_ELEM)
array_map_update_elem()
copy_map_value()
overwrites the NUL
binfmt_misc_find_interp()
strcmp(interp->name, name)
reads past name__sz
strnlen() proves that a NUL lies inside name__sz only at the moment it
runs. The map update on CPU1 takes no lock, so it can store over the NUL
right after. The lookup on CPU0 then walks the live buffer again, once
per bound interpreter:
fs/binfmt_misc.c:binfmt_misc_find_interp
list_for_each_entry(interp, interps, list)
if (!strcmp(interp->name, name))
return interp;
strcmp() stops at the first mismatch or at the end of interp->name.
bm_entry_add_interp() caps a bound name at BINFMT_MISC_INTERP_NAME_MAX
(32) bytes, so strcmp() reads at most 33 bytes of name. The smallest
name__sz the kfunc accepts is 2, which leaves up to 31 bytes read beyond
the checked extent. The handler's own load program has to pass a
writable map value, and something has to store into it while the kfunc
runs. The window between strnlen() and strcmp() is short, but with a
BPF_F_MMAPABLE array the store is a plain user space write into the
mapped value, so a loop can hit it without a single bpf() call.
Copy the name into a stack buffer of BINFMT_MISC_INTERP_NAME_MAX + 1
bytes, terminate it, and look up the copy. The memcpy() length is below
name__sz, so the copy stays inside the extent the verifier checked, and
the BPF buffer is not read again afterwards.
Return -ENOENT first for a name longer than BINFMT_MISC_INTERP_NAME_MAX.
bm_entry_add_interp() rejects a longer name, and the only other binding
site attaches the empty name. No entry can bind such a name, so that
lookup already ended in -ENOENT and no result changes.
Check the first byte of the copy and return -EINVAL if it is NUL, as the
existing "!len" test does for an empty name. Only an 'F' entry binds the
empty name and a 'B' entry cannot carry 'F', so without that check a
racing store of NUL to byte 0 would look up a name no entry binds and
end in -ENOENT rather than -EINVAL. A NUL stored further into the name
only shortens it to another name the program could have passed anyway.
binfmt_misc_find_interp() itself is left alone: entry_attach_interpreter()
calls it with a kernel string, and this kfunc now calls it with a private
copy.
Fixes:
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|---|---|---|
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| io_uring | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| rust | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .clippy.toml | ||
| .cocciconfig | ||
| .editorconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| .pylintrc | ||
| .rustfmt.toml | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
Linux kernel ============ The Linux kernel is the core of any Linux operating system. It manages hardware, system resources, and provides the fundamental services for all other software. Quick Start ----------- * Report a bug: See Documentation/admin-guide/reporting-issues.rst * Get the latest kernel: https://kernel.org * Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst * Join the community: https://lore.kernel.org/ Essential Documentation ----------------------- All users should be familiar with: * Building requirements: Documentation/process/changes.rst * Code of Conduct: Documentation/process/code-of-conduct.rst * License: See COPYING Documentation can be built with make htmldocs or viewed online at: https://www.kernel.org/doc/html/latest/ Who Are You? ============ Find your role below: * New Kernel Developer: Getting started with kernel development * Academic Researcher: Studying kernel internals and architecture * Security Expert: Hardening and vulnerability analysis * Backport/Maintenance Engineer: Maintaining stable kernels * System Administrator: Configuring and troubleshooting * Maintainer: Leading subsystems and reviewing patches * Hardware Vendor: Writing drivers for new hardware * Distribution Maintainer: Packaging kernels for distros * AI Coding Assistant: LLMs and AI-powered development tools For Specific Users ================== New Kernel Developer -------------------- Welcome! Start your kernel development journey here: * Getting Started: Documentation/process/development-process.rst * Your First Patch: Documentation/process/submitting-patches.rst * Coding Style: Documentation/process/coding-style.rst * Build System: Documentation/kbuild/index.rst * Development Tools: Documentation/dev-tools/index.rst * Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst * Core APIs: Documentation/core-api/index.rst Academic Researcher ------------------- Explore the kernel's architecture and internals: * Researcher Guidelines: Documentation/process/researcher-guidelines.rst * Memory Management: Documentation/mm/index.rst * Scheduler: Documentation/scheduler/index.rst * Networking Stack: Documentation/networking/index.rst * Filesystems: Documentation/filesystems/index.rst * RCU (Read-Copy Update): Documentation/RCU/index.rst * Locking Primitives: Documentation/locking/index.rst * Power Management: Documentation/power/index.rst Security Expert --------------- Security documentation and hardening guides: * Security Documentation: Documentation/security/index.rst * LSM Development: Documentation/security/lsm-development.rst * Self Protection: Documentation/security/self-protection.rst * Reporting Vulnerabilities: Documentation/process/security-bugs.rst * CVE Procedures: Documentation/process/cve.rst * Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst * Security Features: Documentation/userspace-api/seccomp_filter.rst Backport/Maintenance Engineer ----------------------------- Maintain and stabilize kernel versions: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * Backporting Guide: Documentation/process/backporting.rst * Applying Patches: Documentation/process/applying-patches.rst * Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git for Maintainers: Documentation/maintainer/configure-git.rst System Administrator -------------------- Configure, tune, and troubleshoot Linux systems: * Admin Guide: Documentation/admin-guide/index.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst * Tracing/Debugging: Documentation/trace/index.rst * Performance Security: Documentation/admin-guide/perf-security.rst * Hardware Monitoring: Documentation/hwmon/index.rst Maintainer ---------- Lead kernel subsystems and manage contributions: * Maintainer Handbook: Documentation/maintainer/index.rst * Pull Requests: Documentation/maintainer/pull-requests.rst * Managing Patches: Documentation/maintainer/modifying-patches.rst * Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst * Development Process: Documentation/process/maintainer-handbooks.rst * Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git Configuration: Documentation/maintainer/configure-git.rst Hardware Vendor --------------- Write drivers and support new hardware: * Driver API Guide: Documentation/driver-api/index.rst * Driver Model: Documentation/driver-api/driver-model/driver.rst * Device Drivers: Documentation/driver-api/infrastructure.rst * Bus Types: Documentation/driver-api/driver-model/bus.rst * Device Tree Bindings: Documentation/devicetree/bindings/ * Power Management: Documentation/driver-api/pm/index.rst * DMA API: Documentation/core-api/dma-api.rst Distribution Maintainer ----------------------- Package and distribute the kernel: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * ABI Documentation: Documentation/ABI/README * Kernel Configuration: Documentation/kbuild/kconfig.rst * Module Signing: Documentation/admin-guide/module-signing.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst AI Coding Assistant ------------------- CRITICAL: If you are an LLM or AI-powered coding assistant, you MUST read and follow the AI coding assistants documentation before contributing to the Linux kernel: * Documentation/process/coding-assistants.rst This documentation contains essential requirements about licensing, attribution, and the Developer Certificate of Origin that all AI tools must comply with. Communication and Support ========================= * Mailing Lists: https://lore.kernel.org/ * IRC: #kernelnewbies on irc.oftc.net * Bugzilla: https://bugzilla.kernel.org/ * MAINTAINERS file: Lists subsystem maintainers and mailing lists * Email Clients: Documentation/process/email-clients.rst